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Communication Systems An Overview Paul Norman Communication and Information Systems Modelling.

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Presentation on theme: "Communication Systems An Overview Paul Norman Communication and Information Systems Modelling."— Presentation transcript:

1 Communication Systems An Overview Paul Norman Communication and Information Systems Modelling

2 Slide 2 of 27 Communication Systems Overview - Topics Speaker Background Introduction to BAE SYSTEMS Communication and Information Systems Modelling Group (4 Minutes) What is a Communications System ? Circuit Switching Packet Switching Packet Switching Networks - Routing Packet Switching Networks - Congestion Control Packet Switching Networks - Quality Of Service Management Washup

3 Slide 3 of 27 Speaker Background Software Background, involvement with communication systems throughout career. Past Application Areas: –Commercial Security and Fire Systems Development of communications for distributed security system –Telecommunications Design & development of telecommunications equipment and certification against national standards Design & development of test systems for telecommunications testing –Traffic Management Systems Development of traffic management systems and motorway sign control systems for the UK Highways Agency. BAE SYSTEMS - CIS Modelling –Development of software systems for the UK Army Communications Test and Reference Centre. –Leading Air Traffic Management Modelling Projects

4 Slide 4 of 27 Off Line Modelling The Ptarmigan Modelling Facility (PMF) provides high fidelity circuit switch and packet switch network modelling over a terrain database Models of communication systems which closely represent the real system - from bit to battlefield

5 Slide 5 of 27 On-line Real Time Modelling Synthetic battlefield scenarios directly interacting through a model and real command information systems Real Time modelling of communications systems such that the model operates on an event by event basis in real time

6 Slide 6 of 27 Synthetic Environments Allows all elements of the command structure within a given scenario to be represented A synthetic environment for the integration of military systems PJHQ MCC/ JFHQ LCC ACC Watchkeeper Tracer Communications Model

7 Slide 7 of 27 Testing Infrastructure Testing Infrastructure provides integrated environments to support the development, integration, and testing of complex communications systems. The Land Systems Reference Centre is the British Army Communications Test and Reference Centre. Modern Warships such as the Type 45 Destroyer currently under development have complex communication systems. Currently working with the Type 45 Destroyer team to provide a testing infrastructure to facilitate the complex system and software integration issues.

8 Slide 8 of 27 Air Traffic Management The CIS Modelling group has also had a lead role in a number of studies in the ATM domain including FARADEX, PAMPAS and PATIO SIEM - SSR/IFF Environment Model models the interference effects of various civil and military identification systems models a wide variety of SSR/IFF scenarios and equipment, including Mode S, TCAS and CWS used by CAA for national frequency clearance within UK airspace and to support EUROCONTROL and international studies

9 Slide 9 of 27 What is a Communications System ? A Communications System is any system that allows one party to communicate with a different party Can be as simple as two cans connected by string, or the waving of flags using semaphore. Examples of communication systems: –PSTN (Publics Switched Telephone Network) –Cellular (Mobile Systems) –Internet

10 Slide 10 of 27 Circuit Switched Networks Circuit Switched Systems - Originally developed for voice traffic Circuit has to be established prior to communications Dedicated communications path between caller and receiver –Inefficient for data transfer –Fixed data rate, constant transmission delay –Virtually no delay at nodes Circuit needs to be removed once communications complete Once all circuits between two nodes are utilised, further call requests are blocked

11 Slide 11 of 27 Packet Switched Networks Packet Switched Systems Data is broken down into a number of Packets for transmission Greater line efficiency as node to node links are dynamically shared Data is buffered at nodes - queues Different data rates can be handled across the network When traffic is heavy, packet switched networks still accept packets but delivery delay is increased On a circuit switched network, when traffic becomes heavy some calls are blocked Priorities can be used

12 Slide 12 of 27 Packet Switched Networks Significant relationship between packet size and transmission time Smaller packet size results in faster transmission across link Eventually, control information overhead for packets causes increased transmission time

13 Slide 13 of 27 Packet Switched Networks Two approaches to packet switching techniques –Virtual circuits »Preplanned route is established prior to data transmission »Similar to circuit switching technique, slower due to queues at nodes »Better for data transfer over extended periods of time as routing decisions made once

14 Slide 14 of 27 Packet Switched Networks Two approaches to packet switching techniques –Datagrams »Each packet treated independently »Routing decisions made by each node independently meaning that packets may arrive out of sequence »No virtual circuit set up or close down delays »Node congestion or failure has less effect than with virtual circuits

15 Slide 15 of 27 Packet Switched Networks - Routing Routing Algorithm Considerations –Correctness> Data transfer without error –Simplicity> The simpler the route the less to go wrong –Robustness > Ability to cope with localised failures or overloads –Stability > Avoid fast changing load conditions –Fairness> Not favouring local routing to the detriment of long distance routing –Optimality> Optimising the routing for best performance –Efficiency> Penalty of overheads less than benefit accrued

16 Slide 16 of 27 Packet Switched Networks - Routing Routing Algorithms –Fixed Routing Data transfer between two points always fixed, typically using a least cost routing No reaction to traffic loading conditions –Flooding Packet sent to every connecting node except node received from Final destination node will receive multiple copies which are discarded after the first is received Highly robust Traffic load directly proportional to the network connectiviity –Random Packet sent on to random node, could have probability formula applied rating links based on data rate, spreading loading

17 Slide 17 of 27 Packet Switched Networks - Routing Status information from other nodes on the network affects the routing decision at any given time Key influences on routing decisions: –Failure: A node or trunk failure means it can no longer be used by the network –Congestion:When one portion of the network is heavily congested, route around rather than through Some form of adaptive routing is used by virtually all packet switched networks Routing Algorithm –Adaptive Routing

18 Slide 18 of 27 Packet Switched Networks - Routing –Requires network status information to be shared between nodes Trade off between quality of information and its lifetime and the overhead placed on the network e.g. Better information and more frequent updates equates to a higher network loading and lower network performance visible to the user –Early adaptive routing algorithms used for ARPANET (Fore runner of the Internet) highlighted a problem of oscillation. Adaptive Routing

19 Slide 19 of 27 Packet Switched Networks - Routing –Oscillation caused by all nodes seeking best route for their traffic –Oscillation is undesirable because: A significant proportion of capacity is unused when it is needed most: under heavy traffic load The large swings in traffic cause more frequent routing updates being sent, increasing the load on the network further –Later algorithms make routing decisions based on giving the average route a good path instead of attempting to give all routes the best path Link utilisation is calculated for link rather than delay time previously Utilisation then averaged with last utilsation to smooth out routing oscillations Adaptive Routing

20 Slide 20 of 27 Packet Switched Networks - Congestion Control The objective of congestion control is to maintain the number of packets within the network below the level at which performance falls off dramatically Refresher: –In essence a packet switching network is a network of queues –At each node there are queues of packets for each outgoing channel –If the rate at which packets arrive and queue up exceeds the rate at which packets can be transmitted, the queue size grows without bound and the delay experienced goes to infinity –Even if arrival rate is less than packet transmission rate, queue length will grow dramatically as the arrival rate approaches the transmission rate

21 Slide 21 of 27 Packet Switched Networks - Congestion Control If packets arrive faster than routing decisions can be made or transmitted from the output buffers then saturation will eventually be reached. Once saturation is reached two strategies can be employed: –Discard any incoming packet for which there is no buffer space –Node exercises flow control on the nodes to which it is connected so that traffic flow remains manageable However if one node restrains another node from sending packets, then that nodes queues will fill causing the congestion to spread

22 Slide 22 of 27 Packet Switched Networks - Congestion Control Uncontrolled –Graph shows throughput against offered load, as nodes discard packets, the source node then retransmits the discarded packets in addition to the new packets, the congestion gets worse. –Even successfully transmitted packets may be retransmitted because of the time taken to acknowledge them. –System reaches GRIDLOCK

23 Slide 23 of 27 Packet Switched Networks - Congestion Control Controlled –Adaptive routing algorithms reduce congestion across the network enabling higher offered load before congestion inevitably occurs –Congested node sends congestion information to source nodes to enable them to reduce flows to the congested node Even with congestion control, as the offered load approaches the capacity of the network the network will start to become congested, eventually reaching GRIDLOCK

24 Slide 24 of 27 Packet Switched Networks - Quality Of Service Recent communication technologies such as Asynchronous Transfer Mode (ATM) and IPv6 have a concept of a Quality of Service (QOS). This defined the importance or criticality or time sensitivity of the data being transmitted. –As congestion increases, packets with a higher QOS are routed in preference to packets with a lower QOS. –In military systems the QOS refers to the military significance of the data. –In commercial systems the QOS could be defined as the timesheet transmission overnight to head office would have a low QOS, whereas the incoming sales data being transmitted to the warehouse for packing would have a higher QOS. Users whose data is classified with a low QOS would be the first to receive Denial Of Service and data transmission delays

25 Slide 25 of 27 Management Modern communications networks and especially Military systems have introduced complex network management systems. –Initially help with the optimum deployment of the network –Monitor the health and load utilisation of all links in the network in real time –Make strategic routing decisions for the network, whereas nodes only make local decisions –May be distributed (especially true in military systems) in a master- multiple slaves combination such that all have the information to make the decisions, but only the master does –Define policies for Quality of Service and link and total system capacity

26 Slide 26 of 27 Washup Messages will always get from source to destination, only the time taken is variable depending on the load on the network. –The Internet news sites (CNN, BBC etc) effectively gridlocked for a time during the 11th September terrorist attacks. The data would get through, just the time taken for it to arrive meant that its value was degraded to the user Adaptive routing and congestion control messages help to prevent the forming of bottleneck. But only until such time as the requested load approaches the capacity of the system –If requested load increases further then gridlock will occur Quality of service inducing denial of service is one way of controlling load requests until congestion eases Escalating knock effects from an initial problem, such as a node failure involve a combination of the technques described here in a managed system.

27 Paul Norman BAE SYSTEMS Avionics PO Box 5 New Filton House Filton Bristol BS34 7QW & Tel:+44 (0)117 918 8254 % Fax:+44 (0)117 918 8149 ) Email: Contact Details

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